1. Field of the Invention
This invention relates generally to the manufacture of glass optical elements, and, in particular, to methods of molding glass optical elements such as lenses and prisms.
2. Description of the Prior Art
In manufacturing glass elements, it is generally necessary that the glass element meet certain criteria in order to be suitable for its intended use. This is particularly true for optical elements. For instance, in selecting a lens intended for use in photographic apparatus where good image-forming qualities are necessary, the nature of the lens surface must be considered. The characteristics of a surface which are important in this regard are known in the art as surface quality and surface accuracy. Surface quality is related to the occurrence of defects such as scratches, digs, pits, voids, "orange peel", etc. on the surface of an element. An element is said to be of "high surface quality" if the number of such defects is sufficiently low so that the element is suitable for its intended use. For instance, in the case of a lens to be used in photographic apparatus, the number of such defects must be low enough so that image forming qualities of the lens are not impaired. It is understood that the number of such defects which can be tolerated depends on the particular element being considered and its intended use.
Surface accuracy, which is usually specified in terms of the wave length of light of a specific color, refers to the dimensional characteristics of the surface, i.e. the value and uniformity of the radius of curvature of the surface. The surface accuracy is generally determined by an interferometric comparison of a surface of the element with a test plate gauge, by counting the number of Newton's rings, and by examining the regularity of the rings. The surface accuracy of an element is often referred to as its fit. The fit of an element is expressed in terms of its power (the number of Newton's rings which are counted) and its irregularity (the difference between the number of rings when counted in perpendicular directions across the fringe pattern). The lower the values of the power and irregularity, the better the lens, or, in other words, the higher its accuracy. Therefore, "high surface accuracy", or "precise fit" refers to a surface which has dimensional characteristics that are extremely close to their design value and are very uniform. For instance, the surface accuracy of a lens to be used in a photographic apparatus is frequently considered high when it has a power of less than six rings and an irregularity of less than three rings.
The manufacture of glass optical elements has conventionally required a series of complex and expensive steps, including molding, grinding, and polishing operations. For instance, in the conventional manufacture of a glass lens, a rough molded glass lens blank is first made by heating a chunk of glass to a softened state and pressing the glass to the desired shape in a metal mold. In some cases, the glass may adhere to the molding surfaces. To prevent this adherence, the mold temperature may be reduced below the glass temperature during pressing. However, this technique may produce an irregular surface called "chill wrinkle", when the hotter glass comes into contact with the cooler mold surface. Another technique is to heat the chunk of glass on a hearth plate prior to molding. A thin layer of a parting agent may be used to prevent the glass from sticking to the hearth plate. The parting agent may become embedded in the rough molded glass surface. When formed with these techniques, a rough molded lens blank does not possess the high surface quality and high surface accuracy necessary for an image-forming lens. Hence, it is necessary to mold a lens blank which is larger than the intended lens element to allow for the removal of material during the subsequent grinding and polishing operations needed to render the lens suitable for use.
Spherical lens surfaces can be generated by rotating the lens blank in a vacuum chuck and grinding the lens blank with a rotating annular tool whose axis is at an angle to the chuck axis. The tool has an abrasive surface including diamond chips. The geometry of this arrangement causes a sphere to be generated wherein the radius is determined by the angle between the axes of the chuck and of the rotating generating tool, and by the effective diameter of the tool. The thickness is governed by the distance the work is advanced into the tool. The surface of the lens blank may be refined further by grinding operations performed with loose abrasive in a water slurry and cast iron grinding tools.
After the grinding operations has been concluded, the lens element can be polished by a process similar to the grinding process. The polishing tool is lined with a layer of pitch and the polishing compound is a slurry of water and rouge (iron-oxide) or cerium oxide. Polishing is continued until substantially all of the grinding pits and scratches are removed from the surface of the lens. Then, the lens shape is checked and corrections are made to assure the proper shaping of the lens.
Following the polishing operation, the lens is centered by grinding the rim of the lens, so that its mechanical axis (defined by the edge of the lens) coincides with the optical axis (the line between the centers of curvature of the two lens surfaces). Lens centering can be done either by known visual methods which are very accurate or by more economical mechanical methods.
It is considerably more complicated to produce nonspherical lens surfaces. The manufacture of precise aspheric lens surfaces requires a combination of exacting measurement and skilled hand correction. One method involves the difficult operation of working a lens blank between centers on a lathe. Aspheric lenses can be made in small production quantities, where high precision is not required, by means of a cam guided grinding rig for generating the lens surface. Thereafter, the troublesome operations of grinding and polishing the aspheric lens surface are performed, the problem being that these operations can easily destroy the basic shape of the lens. Where precise aspheric surfaces are required, it is necessary to make grinding adjustments manually with the concomitant requirements of great delicacy and finesse, the shortcomings of which are apparent.
The expense of existing methods for fabricating glass lenses has led to the limited use of plastic lenses. Plastic has several advantages as a lens material, namely, it is light, shatterproof, and moldable. However, presently available plastics which are practical for use as lens materials such as polystyrene, polycyclohexyl methacrylate, and polymethyl methacrylate, are relatively soft and scratch easily. Moreover, the latter plastic tends to be frequently hazy and sometimes yellowish. Also, plastics usually soften within the range of 60.degree. to 80.degree. C. and their indices of refraction may change in time. Most plastics absorb water and are subject to dimensional change, the latter charactertistic being due to their tendency to cold flow under pressure and to their high thermal expansion coefficient which is almost ten times that of glass. In addition, the high thermal expansion of the plastics causes changes in the indices of refraction of the plastics to an extent ten times that of glass, thus severely hampering the optical performance of the lens.
Thus, glass is clearly a more desirable lens material than plastic, but plastic lenses are considerably easier and cheaper to manufacture than glass lenses because they can be mass produced by molding. However, conventional molding methods have not been found suitable for directly making glass lenses that do not require further preparatory operations. One reason for this is the tendency of heated glass to adhere to some materials and for the glass to remain adhered to the materials after cooling. Thus, one of the prerequisites for producing a suitable lens directly from a mold is that the glass being molded does not permanently adhere to the molding surface. Non-adherence alone, however, is not sufficient, because it has been found that glass will replicate the surface of materials to which the glass does not ahere. For example, glass molded in metal dies has been found to reproduce the grain structure of the metal molding surfaces on the surface of the glass, and such lenses are unsuitable for optical uses without further operations to improve the quality and accuracy of their surfaces. U.S. Patent 3,244,497 discloses the use of extremely thin coatings of refractory materials (approximately half wavelength) to protect a mirror finish metal molding surface and act as a parting agent in a glass molding structure for producing ophthalmic lenses. But, even though surface characteristic tolerances for most ophthalmic lenses are not as stringent as for many optical elements (e.g. 100 ring power may be acceptable), it is nevertheless still necessary to perform additional polishing operations on lens blanks molded in the molding structure disclosed in U.S. Pat. No. 3,244,497 in order to produce even an ophthalmic lens suitable for use. Thus, although it is apparent that in order to directly mold glass lenses the mold surfaces must be of high quality and high accuracy and must not be adhered to by glass, it is equally clear that meeting these requirements does not guarantee that the lens produced will not require further preparatory operations, such as polishing, in order to be rendered suitable for use. The failure of known molding methods to directly produce glass optical elements suitable for use by molding alone has necessitated continued reliance on the time consuming and expensive grinding and polishing operations described above.
Recently, glasslike carbon materials have been developed which have found many applications in the electronics and metallurgy fields. It has been discovered, as disclosed in commonly assigned U.S. Pat. No. 3,900,328 issued Aug. 19, 1975, that these glasslike carbon materials can be used as a molding surface in a mold cavity for directly producing glass lenses which require no subsequent grinding and polishing operations, wherein a heat-softened glass is placed in the mold cavity and pressed to form a lens having a shape generally determined by the shape of the mold cavity. An improved method of molding glass lenses employing these glasslike carbon materials is disclosed in U.S. Pat. No. 3,833,347, issued Sept. 3, 1974, a continuation of U.S. application Ser. No. 93,351, filed Nov. 27, 1970, wherein the portion of glass to be molded is heated while it is in proximity to or in contact with the glasslike carbon molding surface. Another improved method of molding glass into optical elements employing glasslike carbon molding surfaces is disclosed in U.S. Pat. No. 3,844,755, issued Oct. 29, 1974, wherein optical glass in a glasslike carbon transfer chamber is heat-softened and subjected to pressure, thereby transferring the glass through a sprue and into a mold cavity having molding surfaces of glasslike carbon.
While the use of glasslike carbon represents a significant breakthrough in the art of lens fabrication, glasslike carbon possesses several properties which make it a less than ideal molding-surface material. Glasslike carbon is subject to oxidation, is structurally weak, is subject to surface scratching, has a low modulus of elasticity, has low fracture and impact strength, and has low thermal conductivity. All of these characteristics are undesirable in a glass molding material, and tend to limit the usefulness of glasslike carbon molding surfaces. It would be desirable to find other mold materials possessing the favorable glass molding properties of glasslike carbon, but materials which would at the same time possess improved structural and thermal properties.